How to speed up the OpenGL (viewport) rendering?
Firstly I would like to state that OpenGL viewport rendering has nothing to do with Cycles rendering. 

Speed of OpenGL viewport rendering depends heavily on your GPU and in some cases (particle/physics/modifier heavy scenes) also on your CPU. There are certain borders that GPU can't get across whatever the setting may be.

Here are two solutions that might help you though, altough I am not certain what kind of result are you looking for. Both are diminishing the actual result but the speedup is considerable in certain situations.



Disabling the Anti-Aliasing would improve the speed of the OpenGL rendering greatly.





You may also think about using Bounding boxes only, if you are trying camera movement or animating a lot of objects and you want to see how their general location and rotation change over time.